The present invention relates to a collimator, and, more particularly, to a collimator including optical fibers held by a capillary and a gradient index rod lens which is optically connected to the optical fibers.
For example, a conventional dual fiber collimator is assembled by connecting its components by a resin or adhesive. A collimator 100 shown in FIG. 1 is of a type in which a resin is impregnated in an optical path and includes optical fibers 12 and 13 held by a capillary 11 and a gradient index rod lens 14. The end face 11a of the capillary 11, which faces the lens 14, and end faces of the optical fibers 12 and 13 are inclined to prevent reflection and returning of light. The end face 14a of the rod lens 14, which faces the optical fibers 12 and 13, is also inclined. The end face 11a of the capillary 11 and the end face 14a of the rod lens 14 are connected together by an adhesive 15.
The collimator 100 in FIG. 1 has the following disadvantages.
(1) The adhesive 15 in the optical path of the rod lens 14 degrades the resistance of light power of the collimator 100.
(2) Because it is necessary to use an adhesive whose refractive index is matched with the refractive indexes of the rod lens 14 and the optical fibers 12 and 13, the degree of freedom of selection of the adhesive is small.
(3) There is no adhesive whose refractive index is completely matched with the refractive indexes of the rod lens 14 and the optical fibers 12 and 13, and relatively thin though the adhesive layer is, the transmittance of the adhesive is not 100%. Those points lead to light loss.
A collimator 200 shown in FIG. 2 is of an optical-path free type. The end face 11a of the capillary 11 is connected to the end face 14a of the rod lens 14 by an adhesive 16 (or resin) such that the adhesive does not enter the optical path of the rod lens 14. In this case, the end face 11a and the end face 14a may be connected together by inserting a spacer with a proper thickness between those end faces. In case of the rod lens 14 with a diameter of 1.8 mm, for example, the rod lens 14 is connected to the capillary 11 such that the adhesive 16 does not enter the optical path with a diameter of 0.9 mm.
Because the adhesive 16 is present out of the optical path of the rod lens 14 in the collimator 200 in FIG. 2, the collimator 200 does not have the disadvantages of the collimator 100 shown in FIG. 1. It is, however, difficult to form a layer of the adhesive 16 in a gap of several tens of microns between the capillary 11 and the rod lens 14, excluding the optical path portion, while considering the position of the capillary 11. In this respect, the following connecting scheme has been proposed.
Generally, the wettability of adhesives varies depending on a target to be adhered. For example, the wettability is superior with respect to a cleaned glass surface and is inferior with respect to a dielectric thin film, such as an antireflection film. Using this property, the end face 14a of the rod lens 14 and the end face 11a of the capillary 11 where antireflection films are formed are processed as follows.
An etching-resistant film is formed in intraoptical path areas (portions A in FIGS. 3A and 3B) of the antireflection films formed on the end faces 14a and 11a. Thereafter, those areas (portions B in FIGS. 3A and 3B) of the end faces 14a and 11a, which exclude the intraoptical path areas of the antireflection films, are removed by using a strong acid. Then, the etching-resistant film is removed and the adhesive 16 is injected into the gap between both end faces 14a and 11a. This scheme needs a treatment of the rod lens 14 and the capillary 11 with a chemical, such as a strong acid, and the treatment may damage optical parts, such as the rod lens 14.